The present invention relates to the electrostriction and electrostatic charge induced striction of electroactive polymers and the application of this phenomenon to transducers. Electroactive polymer based transducers are activated by application of an electrical field. The response of the material (embodied by strain) depends on this applied field and on the boundary conditions imposed by the environment. The dependency of strain on the applied field is due to the combined phenomena of electrostriction and/or the Maxwell stress effect. In the case of electrostatic charge induced striation, strong dependence of the strain response to the boundary conditions arises since the actuating medium behaves as an elastic body under load to which the Maxwell stress effect is one of several boundary conditions. Electrostriction is a coupling effect between the strain and the square of the applied electric field. It is observed in ferroelectric polymers due to crystalline phase transitions induced by the applied electric field.
The response in any particular direction can be tailored using anisotropic boundary conditions or by stretching the polymer to impose aniostropic mechanical properties about the operating point. In the case that the electroactive polymer is used above the glass transition temperature, the stretched condition must be rigorously maintained during operation. In this case, the boundary conditions can be fixed by careful application of frames and supporting elements. In any comprehensive model of the actuator, these elements must be considered. The boundary conditions often impair the strain of the actuator in more than the intended direction. This effect is often evident when soft frames are used on the film to prevent crack propagation, but also oppose the material in the direction of actuation. Many of the structural reinforcements that are used to prevent device failure or maintain a stretched configuration have structural rigidity that cannot be entirely controlled in every direction. The rigidity of these elements may have an adverse effect on the performance of the transducer and in some instances may cause device failure. Beyond the boundary, near the intersections of free and fixed boundary conditions there exists a boundary phenomenon over which the local response of the electroactive polymer can vary dramatically in comparison to the majority of the material. Many of these conditions lead to local failure of the dielectric electroactive polymer transducer (DET) during operation and should be considered during the design and fabrication of the actuator.
The act of stretching the electroactive polymer can significantly improve its strain response [R. Pelrine, R. Kombluh, Q. Pei, and J. Joseph, Science, 287, 836, 2000]. In the case of electrostatic charge induced striction, anisotropic stretching of the electroactive polymer can improve and/or tailor actuator performance in three different ways: create anisotropic constitutive mechanical properties, modify the boundary conditions, and improve the dielectric strength of the material.
The creation of anisotropic constitutive mechanical properties can be used to modify the output of the actuator in a similar way to imparting anisotropic boundary conditions on it. Actuators comprised of materials that experience extreme strain hardening (such as silicones) can benefit greatly from anisotropic prestretching. It has been demonstrated in the literature that high transverse prestretching of such materials can be used to optimize their axial strain response.
The dielectric strength of the material can also be improved with prestretch. This in turn means that higher fields can be applied to the film before dielectric breakdown occurs. This effect has been reported in the literature for polyacrylate materials [G. Kofod, R. Kombluh, R. Pelrine, and P. Sommer-Larsen, SPIE Proceedings 4329, 141, 2001]. One existing hypothesis states that the increase in the dielectric strength can be attributed to the extension of polymer chains during prestretching in a direction perpendicular to the applied field. This decreases the energy that free electrons can transfer to the chains as they pass from one electrode to the other and therefore increases the energy that these electrons can possess before they lead to avalanche breakdown. The dielectric strength may also increase due to thinning of the polymer film with stretch in the case that the electroactive polymer experienced thermal dielectric breakdown.
The existing mechanisms used to maintain prestretch in the electroactive polymer during operation are bulky and as such add significant weight and size to practical devices [R. Pelrine et al., U.S. Pat. No. 6,376,971 issued Apr. 23, 2002; R. Peirine et al., U.S. Pat. No. 6,343,129 issued Jan. 29, 2002]. Furthermore, the current state of the art for practically realizable mechanisms for maintaining prestretch interfere with the strain response of the electroactive polymer and as such have limited the response of practical devices to strain levels much below those demonstrated in laboratory settings.
Accordingly it would be advantageous to provide a dielectric elastomer transducer in a lightweight, compact package. Further it would be advantageous to provide a dielectric elastomer transducer having high transverse prestretch polymeric material. It would be advantageous to utilize stiff support material to such that the polymer stretched in the circumferential direction while also provide a means of interfacing the device with its environment and minimizing interference with the axial movement of the transducer. Furthermore, it would be advantageous to optimize mechanical properties of the support materials to further enhance performance of the transducer by increasing the maximum strain output of the device, increasing the stiffness of the device or Introducing biaxial stretch to the electroactive polymer layers.